The present invention generally relates to a zoom lens system having temperature compensation function and a video camera using the same. In particular, the present invention relates to a compact, high zoom ratio and low-cost zoom lens system with temperature compensation function using a plastic lens, and a video camera using the zoom lens system.
Conventionally, in an optical system for a video camera, such as a camcorder or a digital still camera, a zoom lens system that has four lens groups is well known. The system includes, in order from an object side, the first lens group that has positive refractive power, the second lens group that has negative refractive power, the third lens group that has positive refractive power, and the fourth lens group that has positive refractive power. The first lens group and the third lens groups are both fixed. The second lens group moves along an optical axis for zooming, and the fourth lens group moves along the optical axis for focusing and for compensating image surface blur caused by zooming.
Japanese patent laid-open publication No. H09-311272 discloses an optical system that reduces the cost by using five plastic lenses in a lens system of total ten lenses. In this system, variation of focal position of the entire optical system caused by temperature change is reduced by canceling focal length change caused by temperature change. This is accomplished by combining a plastic convex lens and a plastic concave lens. However, if the plastic lens is to be used in a compact optical system, problems occur such as it is difficult to keep the Petzval sum small and the coma aberration becomes big. This is because the refractive index of plastic lens is lower than that of glass lens.
Another Japanese patent laid-open publication No. H05-93832 discloses a zoom lens system that has a detection means for detecting temperature of the lens system and outputting an electrical signal based on the detected temperature, and a control means for driving a lens by the electrical signal in order to compensate divergence of the focus caused by temperature change of a plastic lens by moving the lens. This system includes a detection means and a control means; therefore, it is inevitable that it will be expensive.
In addition, Japanese patent laid-open publication No. H10-293261 discloses that deviation of an image forming position due to the temperature characteristic of a plastic lens is compensated by expansion or contraction of a lens barrel in accordance with temperature change.
Moreover, it is desirable to utilize an aspherical lens in order to achieve a compact and high-performance zoom lens system.
As a means for molding an aspherical surface, a molding method for glass lens, plastic injection molding, and what is called a hybrid-lens that is formed by putting a plastic layer on a spherical glass lens, are well known. An aspherical glass lens may require a molding method performed at high temperature, and therefore life duration of the expensive die molding is short. For that reason, cost of an aspherical glass lens is about three times as expensive as that of spherical glass lens. Regarding hybrid type lens, there is no restriction in the kind of glass available, but the hybrid lens requires glass lens and a mold for plastic layer. Therefore, a hybrid lens is also expensive in cost though it is not so expensive as glass molding. A plastic lens is inexpensive compared to glass and hybrid. However, the refractive index of a plastic lens is low, and types of available plastic lenses are limited. Besides, if a plastic lens is to be used in a compact optical system by a strengthening power arrangement, coma becomes big, Petzval sum is difficult to keep small, and is easily influenced by temperature change.
A zoom lens system includes a first lens group, a second lens group that moves along an optical axis, a third lens group that has a positive refractive power, negative temperature coefficient of refractive index, and a predetermined focal length, a fourth lens group that moves along the optical axis, a first lens barrel which extends from the first lens group to the third lens group, and a second lens barrel which extends from the third lens group to an image pickup element, wherein the first and second lens barrels each has a respective predetermined linear expansion coefficient, and variation of image location caused by temperature change in the first, second, third and fourth lens groups is cancelled by image location variation caused by expansion or contraction of at least one of the first lens barrel and the second lens barrel.
In addition, the third lens group includes a plastic lens and the plastic lens satisfies the following conditional expression:
5.0 less than f3/fw less than 7.0 
where f3 is a focal length of the plastic lens of the third lens group, and fw is a focal length of the zoom lens system at the wide-angle end.